This invention is concerned with transients in optical fibre amplifiers.
In an optical transmission system containing one or more optically pumped optical amplifiers in its transmission path from optical transmitter to optical receiver, the occurrence of transients in such amplifiers, such as transients in the power level of the input data signal, can produce artifacts that appears as bit errors at the receiver.
A signal with too high an optical power is subject to non-linear effects in an optical fibre transmission path, such as Self-Phase-Modulation, that can seriously degrade the signal. This causes bit errors or loss of frame in the signal. These non-linear effects are especially severe at bit rates at and above 10 Gbit/s. The onset of the non-linear degradations can be quite sharp, so that only one or two dB of increase in power level can push a signal from optimum performance to a failed state. Conversely, a signal with too low an optical power is subject to noise degradations after suffering further attenuation in the transmission path.
Transients in the power of an optical signal can move that signal away from its optimum power level towards too high or too low a power level. Power margins must be allocated in the design of the transmission system so that, during a worst case transient, in combination with other worst case conditions, the bit error rate remains within specification. In setting the margin, making allowance for these effects of transients reduces the performance that would otherwise be available, performance that could for instance otherwise be used for increasing amplifier spacing.
Even within an appropriate power range, power transients can cause bit errors. These are liable to occur for instance when the transient is faster than the automatic gain control in an amplifier at the receiver, thereby causing a momentary overload of the receive electronics. The consequential distortion produced by such overload can produce bit errors. Moreover, during a transient, the electrical signal, or eye, at the 0,1 decision circuit will be larger, or smaller, than anticipated. This places the decision threshold at the wrong location in the eye, which causes errors.
A further undesirable feature of amplitude transients is that they can produce phase transients in clock recovery circuits and so contribute to jitter, which in turn can increase bit error rate.
Erbium doped fibre amplifiers can cause amplitude transients when used for simultaneously amplifying several wavelengths. Consider the simple example of such an amplifier amplifying two wavelengths. If one wavelength is removed while the amplifier pump is held constant, then the output power at the other wavelength will increase by 3 dB. The speed of this transient is determined by the pump power and by the response of the erbium doped fibre, and is measured in microseconds.
Ways in which the gain of optical amplifiers can be controlled are well known, and examples include U.S. Pat. Nos. 5,274,496 and 5,247,529.
European Patent Application EP 0 828 357 discloses, in respect of an amplifier that is amplifying signals in different signal bands, controlling the pump power in a manner that prevents the output power in any one of these signal bands from exceeding a given threshold. This will operate to remove long-term symptoms of a change in power level, but is generally too slow to suppress the onward transmission of micro-second or milli-second transients.
The onward transmission of transients can be suppressed by providing an optical amplifier with positive feedback to cause it to lase at some wavelength not being used for signal transmission. This clamps the gain of the optical amplifier at the lasing wavelength, and therefore also clamps the gain at all other wavelengths in the gain spectrum. However such an approach requires the provision of significant extra pump power, and this is an undesirable expense. Additionally there is the disadvantage that the gain clamping provides specific values of gain at the signal wavelengths, and these values may not match the needs at that specific amplifier.
At the 22.sup.nd European Conference on Optical Communications--ECOC '96, Oslo, in a paper (TuD. 1.3) given by R E Tench entitled, `WDM optical amplifiers--Design and Applications`, fast electronic gain control in a two-stage amplifier was described for combating gain shifts resulting from the adding and dropping of signal channels. At the same conference, in a paper (TuD.2.2) given by K Aide et al entitled, `Bi-directional Repeatered Transmission over 400 Km using Gain Stabilized Linear Repeaters`, and also in U.S. Pat. No. 5,475,529, there is described using the level of Amplified Spontaneous Emission (ASE) radiated laterally from the erbium fibre to drive a gain control circuit. In U.S. Pat. No. 5,506,724 there is described a similar approach, but in which it is the longitudinal ASE directed out of the amplifier input that is employed for gain regulation.
The response of an erbium doped amplifier has a pole that moves about the region of 300 Hz to 1 kHz, depending upon the input, output, and pump powers. These powers vary with the specific system application. For a stable control system with a bandwidth in the region of this pole, a zero must be closely matched to the pole. Because the location of the pole varies, especially during an optical transient, a static zero will not closely match the pole. If the bandwidth of the control loop is kept less than the region of this pole then the loop will not respond to fast transients. Classic linear adaptive control methods such as Kalman filtering are not fast enough because the pole moves rapidly during the transient, rather than drifting relatively slowly.
If the bandwidth of the loop is made very large, stability can be obtained, for example by using the inherent pole as the only pole in the loop. This fast loop will respond quickly to transients. However, such a wide bandwidth loop will react strongly to noise or artifacts in the measurement of the gain. Such an artefact can be created by the pattern variation in the data carried by the input signals when passed through the high-pass filtering effect of the optical amplifier.
European Patent Application EP 0 849 893 discloses an approach to the solution of the problem of transients that are liable to occur as the result of switching in or dropping out of one or more wavelength multiplexed signal channels being amplifier by an amplifier. The occurrence of these transients is suppressed by arranging for the power levels in channels being brought into service to be slowly faded in, and similarly for those in channels being taken out of service to be slowly faded out. An optical system can be managed so that all channel additions are predicted, thereby enabling appropriate fade-in provision to be made. The same is of course intrinsically not true in respect of any sudden unpredicted failure of a channel. The disclosure does however describe how to add power in a dummy signal wave length to compensate for such a drop. However that approach is relatively expensive, and uses a potentially valuable portion of the gain spectrum for the dummy signal wavelength which otherwise could have been used for real signal traffic.
Thus there is not a really efficient method known for compensation of sudden power drops where that method allows an optical amplifier to function stably in a realistic range of system applications, and the amplifier does not react excessively to small perturbations.